1. Field of the Invention
This invention relates to an electronic package assembly of the type having an encapsulated device, and also to methods for assembling such packages. More particularly, the invention relates to electrical interconnection of an external package lead to the device mounting pad of such an electronic assembly.
2. Description of the Prior Art
In the microelectronic device art, it is known to encapsulate silicon chip integrated circuits in a recess formed in a surface of a ceramic package substrate. A pattern of electrical leads is screen printed with conducting ink or paint onto the substrate surface and arranged to provide interconnecting paths between respective terminals of the integrated circuit and the external package leads. The mounting of the silicon chip within the recess is facilitated by painting over the lower portion of the recess a bonding material which can adhere to both the bottom of the chip and the bottom portion of the recess with which it makes contact. If a conventional gold-glass binder coating or pad is used, the device mounting step, in addition to mechanically securing the chip in the package, provides an electrically conductive connection to the base of the chip. This also allows the heat generated in the integrated circuit to be dissipated by conduction into the package.
Before mounting the silicon chip within the recess, the screen printed conductor mating with the external package ground lead is electrically connected to the device mounting pad. This provides the ground connection to the encapsulated device. The chip is then attached to the pad and the terminal points of the integrated circuit are wire-bonded to respective interconnecting paths on the surface of the ceramic package substrate. The last step in the fabrication of the electronic package assembly comprises attaching a lid over the substrate so as to hermetically seal the silicon chip within the package.
In the prior art, considerable difficulty has been encountered in making a sturdy and reliable ground lead connection between the screen printed conductor on the surface of the package substrate and the device mounting pad at the bottom of the recess. Present techniques, such as screen printing down the step between the substrate surface and the recess, or wire-bonding, provide a low yield of acceptable devices in a mass production operation, in addition to requiring a separate time consuming and costly operation in the fabrication of the package.